Keep the colleague's microbu-esp32c5 tree in this repository

obu-firmware builds against vanetza-idf from microbu-esp32c5/external, but
that tree was gitignored, so a clone of this repository could not build the
firmware it ships. It is now committed here as ordinary files in its own
folder, microbu-esp32c5/: the colleague's commit cf4b99f plus the V2X2MAP
bridge's signature verification (--trust) used on the bench. Nothing is
fetched from or pushed to the colleague's repository; this repository and
its remotes carry everything. The folder's own .gitignore keeps build output,
downloaded components and private key material out, as it did there; the
committed file set is identical to that repository's tracked files.

The ESP32-C5 is still flashed from obu-firmware/, which only takes
vanetza-idf from microbu-esp32c5/, so the two stay separate folders.
FLASHING.md says how to take a newer version of the colleague's tree (copy
it over the folder, rebuild, test, commit).

microbu-esp32c5/external/vanetza-idf/cmake/FindCapnProto.cmake

@@ -0,0 +1,14 @@
+# FindCapnProto.cmake – thin wrapper around the upstream CapnProtoConfig.cmake
+#
+# The upstream CapnProtoTargets.cmake unconditionally calls add_library(... IMPORTED)
+# and will therefore fail on repeated find_package() invocations in the same configure run.
+# This wrapper short-circuits the second call when the targets already exist.
+
+if(NOT TARGET CapnProto::capnp)
+    # Forward to upstream CapnProtoConfig.cmake.
+    # Always QUIET, find_package_handle_standard_args below controls options and output
+    find_package(CapnProto CONFIG QUIET)
+endif()
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(CapnProto CONFIG_MODE)